Lithium ion secondary batteries are widely used in small portable device applications such as mobile telephones or notebook personal computers. Non-graphitizable carbon can be doped and de-doped with lithium in an amount exceeding the theoretical capacity of 372 mAh/g of graphite and also has excellent input/output characteristics, cycle durability, and low-temperature characteristics, so it has been developed (Patent Document 1) and used as a negative electrode material for lithium ion secondary batteries.
In recent years, lithium ion secondary batteries have been increasingly developed and put into practical use for automobile applications due to increasing concern for environmental issues. In automobile applications, there is a particular demand for input/output characteristics, cycle durability, and low-temperature characteristics in addition to a large doping capacity and de-doping capacity (that is, charge-discharge capacity), and non-graphitizable carbon is preferable in that it is excellent with regard to these points.
Among automobile applications, in the case of a hybrid electric vehicle (HEV), there is a demand for high input/output characteristics for repeating the supply and receipt of a large current in a short amount of time at the time of a decelerating or light-load running mode or at the time of regeneration by braking. In order to obtain such a nonaqueous electrolyte secondary battery, a non-graphitizable carbonaceous material with a further reduced resistance has been proposed as a material for a negative electrode (Patent Document 2).
However, although the nonaqueous electrolyte secondary battery using a carbonaceous material described in Patent Document 2 has improved input/output characteristics, the charge-discharge capacity is reduced. For example, a nonaqueous electrolyte secondary battery used in an electric vehicle (EV) driven by a motor alone must have not only a long life and excellent input/output characteristics, but also a high energy density. Therefore, there is a demand to improve rate characteristics (output characteristics) while maintaining a discharge capacity greater than the theoretical capacity of graphite (372 mAh/g).